1. Field of the Invention
The present invention relates to an apparatus for detecting the amount of remaining developer, e.g., for use in an image forming apparatus relying on the electrophotographic method.
2. Related Background Art
In recent years, there have appeared image forming apparatuses relying on the electrophotographic method such as a copying machine, an electrostatic printer, and a facsimile apparatus, wherein a developer container, a photosensitive drum, a primary charger, a developing unit, and a cleaner are integrated as a process cartridge which can be detachably mounted on the image forming apparatus. The maintenance operation is facilitated by replacement of the entire process cartridge.
There are possibly several replacement factors of this process cartridge, but one of such factors involves the case where the developer within the developer container has been used up.
The developer container is prefilled with the developer in an amount determined by the amount of developer used per revolution on the photosensitive drum and the number of revolutions of the photosensitive drum over its life, whereby the developer remaining will decrease when the image forming apparatus is used.
Hence, developer presence/absence detecting means is typically provided in the main body of the image forming apparatus to detect whether or not developer is present, thereby determining the replacement timing of the process cartridge.
An example of such detecting means is illustrated in FIG. 4, wherein a process cartridge 28 is detachably disposed in the apparatus main body, and wherein a photosensitive drum 21 and a developing unit 23 are accommodated integrally inside the process cartridge 28. The developing unit 23 is comprised of a developing portion 23a for delivering the developer T toward the photosensitive drum 21 and a developer storing portion 23d for storing the developer T within a hopper to supply the developer T into the developing portion 23b with the agitation of an agitator 23c. An optical path 50 made of a transparent material extends vertically through the process cartridge 28 comprising the developer storing portion 23d, and detecting means 52 including an attached light emitting portion 52a and a light receiving portion 52b across the process cartridge is placed in the apparatus main body facing the optical path 50 to detect the developer T.
As the developer T within the developer storing portion decreases, light emitted from the light emitting portion 52a of the developer detecting means 52 will more likely reach the light receiving portion 52b. When the light receiving level exceeds a predetermined level, the depletion of developer is detected.
This light emitting portion 52a uses an LED and the light receiving portion 52b uses a phototransistor or a photodiode.
There is known one method of processing the output signal from the light receiving portion 52b to detect the depletion of developer, which method employs a microprocessor unit (MPU) of one chip containing a ROM and RAM.
The MPU is a processor which performs digital control and operation control using a group of instructions stored in a read-only-memory (ROM), including the transaction with a read-write-memory (RAM).
In the case where the light receiving portion 52b uses a photodiode, upon the light emitted from the light emitting portion 52a reaching the light receiving portion 52b, a light receiving current will flow through the photodiode. This light receiving current is converted into a voltage by a current-to-voltage converter, and the voltage is passed into an AD converter for digitization. This digitized light receiving level information is input into the MPU and compared with time or voltage information to determine the presence or absence of developer T (i.e., the state in which the developer remains in an amount sufficient to record on tens of sheets of transfer medium P) which is contained in the group of instructions stored in the ROM, to detect the replacement timing of the process cartridge 28.
A one-chip microcomputer is constructed by enclosing at least a RAM, a ROM having the operation programs stored therein, and a central processing unit (CPU) into one package.
Accordingly, the developer detecting means as above described has the disadvantage that when the information for determining the presence of developer is stored within the ROM of the one-chip microcomputer, the one-chip microcomputer must be reconfigured by rewriting the information within the ROM if there is a need for changing that information, thereby resulting in more cost and time.
On the other hand, if the presence or absence of the developer is determined based on the current flowing through the photodiode on the light receiving side, it is requisite that the remaining amount of developer may correspond to the photoelectric current flowing through the photodiode on the light receiving side in order to enhance the detection accuracy.
That is, it is necessary that a state of constant transmittance may correspond to the photoelectric current passing therethrough.
Herein, it is noted that the amount of luminescence produced when passing current through the light emitting element differs considerably between individual light emitting elements. Also, the conversion efficiency in converting the light received by the light receiving element into photoelectric current varies between individual light receiving elements.
In order to correspond the remaining amount of developer to the photoelectric current, it is necessary to adjust the amount of luminescence or the degree of amplification of the amount of light received.